A tunnel boring machine (“TBM”) is an excavation apparatus that is typically used to bore a tunnel through soil and rock strata. A conventional TBM produces a smooth circular tunnel wall, typically with minimal collateral disturbance. A breakthrough that made TBMs efficient and reliable was the invention of the rotating head with rotatable cutter assemblies, developed by James S. Robbins. Initially, Robbins' TBM used strong spikes fixedly mounted to the rotating head, but the spikes would break frequently. He discovered that by replacing these grinding spikes with longer lasting rotating disc cutter assemblies this problem was significantly reduced.
Modern TBMs therefore typically employ rotating heads with disc cutter assemblies that are rotatably mounted to the head. The head is urged with significant force against the target surface such that at least some of the cutter assemblies engage the surface. As the head rotates, the cutters fractionate, crush, and/or loosen materials, which are transported away by the TBM. As the loosened material is removed, the TBM progresses to bore the tunnel.
Diverse ground conditions are encountered in the excavation of some tunnels. Sand, marl, limestone, clays, and chalk may all be expected. At times, several types of ground may be encountered simultaneously. The disc cutter assemblies typically must operate in extreme conditions and must function reliably under high loads. For example, the cutter disc or blade may exert in excess of 75,000 pounds force normal to a rock face.
The water table along a tunnel boring trajectory may also vary considerably. In some applications, TBMs encounter highly saturated and flowable materials. When encountering loose and/or saturated soil conditions, the hydrostatic pressures on the cutter assemblies can be significant. If dirt or other foreign matter gets into the cutter bearing assembly, the cutter assembly may seize, requiring the user to repair or replace the cutter assembly before continuing. Cutter assemblies are provided with a durable and rugged seal to avoid the incursion of dirt into the bearing assembly. However, if the hydrostatic loads across the seals become sufficiently high, the seals may be breached.
There remains a need for improved sealing mechanisms to prevent the incursion of dirt and other foreign matter into the cutter assemblies of tunnel boring machines operating in conditions of high hydrostatic pressures.